The present invention relates to an air-fuel ratio feedback controller for controlling an A/F ratio of air-fuel mixture to be supplied to an internal-combustion engine based on output from an A/F ratio sensor provided in an exhaust system of the engine.
In A/F (air-fuel) ratio control of an engine, calculation of an A/F ratio feedback coefficient has been performed based on output from an A/F ratio sensor, such as an O2 sensor, provided upstream of an exhaust gas cleaning catalyst. The A/F ratio is increased or decreased repeatedly within a narrow range centered on a theoretical A/F ratio. The A/F ratio feedback coefficient is a coefficient used for calculating fuel injection time of a fuel injection device (injector) and is determined based on driving conditions.
Japanese Examined Patent Application Publication (Kokoku) No. 7-92008 describes a proportional integration control of the A/F ratio feedback coefficient. A proportional constant at the time of shifting the coefficient as well as the period from the time fuel supply to the engine has been changed to the time switching of the A/F ratio between rich and lean is detected by the A/F ratio sensor is predicted based on a present operating state of the engine. Integration constant in the present integration control is determined from both the proportional constant and the period thus predicted. After the A/F ratio feedback coefficient in the present integration control phase is increased or deceased according to the integration constant, the A/F ratio feedback coefficient in the next proportional control phase is increased or decreased with the predicted proportional constant. The variation range and the cycle of change of the A/F ratio are reduced and the A/F ratio rapidly converges to the stoichiometry or theoretical A/F ratio.
According to such a conventional scheme, in a case that reaction delay time of an O2 sensor increases due to such causes as deterioration of the O2 sensor because of a secular change, an A/F ratio F/B coefficient KO2 may greatly change before the output from the O2 sensor actually reverses. This is because KO2 will change at a large gradient of an integration term calculated based on the reaction delay time TRL or TLR of the O2 sensor, which is shorter than the real delay time.
Accordingly, the A/F ratio may move out of a cleaning window width of a ternary catalyst.
In order to solve such problems, the present invention provides an A/F ratio controller for an internal-combustion engine. In accordance with one aspect of the invention, the controller comprises A/F ratio detector provided in an exhaust system of an internal-combustion engine for detecting an A/F ratio of exhaust gas. The controller further comprises A/F ratio feedback coefficient calculator for calculating an A/F ratio feedback coefficient by proportional term control and integration term control. The controller includes a timer for setting length of time for the integration term to be carried out in accordance with an operating state of the internal-combustion engine. The controller includes an integration term calculator for calculating the integration term based on the time set by the timer, and proportional term setting means for setting a proportional term for shifting the A/F ratio feedback coefficient.
The controller further comprises a deviation detector for detecting a deviation between a first A/F ratio feedback coefficient before the integration term is carried out and a second A/F ratio feedback coefficient after the integration term is carried out and the coefficient is shifted by the proportional term set by said means for setting the proportional term. The controller includes means for correcting, based on the deviation, the length of time for the integration term to be carried out.
With reference to an example of the A/F ratio control phase shown in FIG. 3B, the basic concept of the technique according to the present invention will be described. At the time of reversal of the A/F ratio from rich to lean and from lean to rich, the time required to subsequently carry out the integration term is corrected according the invention. The correction is made based on a deviation between the A/F ratio feedback coefficient KO2SRL that is the coefficient before the integration term IL is carried out and the coefficient KO2SLR that is the coefficient after the integration term IL is carried out and a shift KO2WL is added.
According to the invention, the integration term is calculated based on the time thus corrected. Therefore, when a reaction delay due to a secular change or the like occurs in the A/F ratio detector, the value of the integration term becomes smaller in accordance with the delay. Thus, excessive change of the A/F ratio feedback coefficient before the output of the A/F ratio detector reverses is avoided. Thus, even when the reaction delay time changes due to deterioration or the like of the A/F ratio detector, it is possible to decrease disturbances in the A/F ratio feedback control.
In accordance with another aspect of invention, the controller further comprises means for learning a reaction delay in the A/F ratio detector based on the deviation, and means for determining deterioration of the A/F ratio detector when a learned value of said means for learning reaches an upper limit value.
According to the invention described above, deterioration of the A/F ratio detector can be detected, which is an important data source in the A/F ratio feedback control, in a normal process of the A/F ratio feedback control.